1. Field of the Invention
This invention relates to a photoelectric conversion element and a photoelectric conversion device for converting high speed optical pulses into high frequency waves or electromagnetic waves in optical signal processing, optical communication, high frequency wave signal processing or high frequency wave signal communication.
2. Description of the Related Art
In the field of communication, basic trunk communication networks, as well as communication networks to each user have tended to be replaced with optical communication in recent years. However, in the communication terminals represented by portable remote terminals and portable telephones, high frequency wave communication will remain important in the future, and the entire communication networks have to be in the form where optical communication networks and high frequency wave communication network are present together.
At present, in the optical communication and the high frequency wave communication, signals used therein are once converted into electric signals and amplified or switched. High speed optical modulators or laser optical modulation technique are also present. Further, there is also adapted a technique of applying a high speed pulse light at femtosecond order to semiconductors or oxide superconductors thereby generating electromagnetic waves at a wavelength of about THz.
Generation of THz electromagnetic waves by the semiconductors or oxide superconductors is promising in a high speed region of 100 GHz or higher. According to the report by Sakai, et al., GaAs thin film optical switches grown at a low temperature are promising optical switches having a carrier life of 1 ps or less.
Generally, when a bias voltage is applied in most of semiconductors, the energy band near the surface is in a bent state, which forms a depletion layer to cause surface electric fields. When laser pulses at the femtosecond order are applied to the surface of the semiconductor forming the surface electric fields, electrons and holes induced by the laser beam are accelerated by the electric fields on the surface of the semiconductor, by which a surge current flows to generate THz electromagnetic waves by dipole radiation. The electric field component of the electromagnetic waves is in proportion with the time differential of the surge current and in proportion with the quadratic time differential of the polarization ratio on the surface of the semiconductor. In InP or GaAs having a wide band gap, the induced surface electric fields are large and, further, THz pulse waves by optical rectification of incident optical pulses by quadratic non-linear optical effect near the surface are also radiated simultaneously.
On the other hand, in InSb or InAs having a narrow band gap, induced surface electric fields are not large in which a surface current flows due to the difference of the diffusion rate between electrons and holes formed upon excitation of the surface by optical pulses referred to as a photo-dember effect, which causes THz pulse waves and radiation of THz pulse waves due to optical rectification effect also occurs.
However, in optical modulators or laser optical modulation technique, the upper limit for the speed is about several tens of GHz and high speed response in a several hundreds of GHz band or several THz cannot be obtained. Further, in the semiconductor optical switches, only the optical purpose with the incident wavelength of about 800 nm can be used and they are not applicable for a light at the wavelength of about 1.5 xcexcm, which is an actual communication band.
Accordingly, this invention intends to provide a photoelectric conversion element and a photoelectric conversion device of directly converting high speed optical pulse signals in a communication band into signals of high frequency waves or electromagnetic waves.
The foregoing can be attained in accordance with this invention. That is, a photoelectric conversion element according to this invention includes a substrate and a light sensor disposed on a surface of the substrate to receive high speed optical pulse signals and convert them into high frequency waves, in which the light sensor includes at least carbon nano-tubes.
According to the photoelectric conversion element of this invention, when optical signals including optical pulses at a wavelength, for example, of equal to or larger than 1 xcexcm and equal to or smaller than 2 xcexcm or less are applied to the carbon nano-tubes, they are converted into high frequency waves or electromagnetic waves and radiated. Further, according to the photoelectric conversion element of this invention, high speed response is possible to, high frequency wave optical signals at 1 MHz or higher, as well as 10 THz.
More specifically, when a pair of electrodes is connected to both ends or the periphery thereof of the carbon nano-tube, respectively, high frequency waves or electromagnetic waves are formed between the electrodes. On the other hand, in a case where the electrodes are not connected or the electrodes are not incorporated in an electric circuit, the high frequency waves or electromagnetic waves converted from the high speed optical pulse signals are further radiated into air. Although the principle is not apparent, it is estimated that when InAs having a narrow band gap and of high mobility are formed into fine particles, the response speed to the light is improved and electromagnetic waves in accordance with the high speed optical pulses are radiated. It is further estimated that electro-optical conversion of high speed pulse signals can be attained by the efficient absorption of the electromagnetic waves to the carbon nano-tubes.